1. Field of the Invention
This invention relates to a surface fastener in which a plate-like substrate and a multiplicity of hooks are continuously and integrally molded by extrusion, and more particularly to a molded surface fastener in which the plate-like substrate is thin and flexible and has adequate toughness in all directions.
2. Description of the Related Art
A hooked surface fastener has been known long since in which hooks are formed by cutting loops of monofilaments woven into a woven cloth. With this type of surface fastener, softness of the woven cloth and softness of the monofilaments combine to make a very smooth touch when the hooks come into and out of engagement with loops of the companion fastener member. Additionally, since the monofilaments forming hooks are treated with drawing process, they are excellent in toughness against pulling and bending though small in cross-sectional area. Further, since the hooks can be formed in high density depending on the structure of the woven cloth, this type of surface fastener has a high engaging rate and hence can survive in repeated use. However, with this woven-cloth-type surface fastener, partly since the quantity of material is large and partly since a large number of process steps are required, it is difficult to reduce the cost of production.
To this end, an alternative molded surface fastener has been developed in which the substrate and the hooks are simultaneously and integrally molded by extrusion. The molding technology for this kind of surface fastener is disclosed in, for example, Japanese Patent Publication No. SHO 48-22768, Japanese Patent Publication No. SHO 52-37414, U.S. Pat. No. 3,312,583, and International Patent Japanese Publication No. HEI 1-501775.
According to the technology disclosed in Japanese Patent Publications Nos. SHO 48-22768 and SHO 52-37414, the substrate and the hooks can be molded integrally and continuously by extrusion. This molding method comprises steps of arranging a large number of mold discs and a large number of spacer plates alternately one over another to form a laminate drum, extruding a thermoplastic resin in a molten form onto the peripheral surface of the drum in rotation to force the resin into hook-forming cavities of the mold discs, compressing the resin painted over the drum surface to form a substrate, cooling the substrate and hooks, retracting the spacer plates radially inwardly, and peeling a completed belt-like surface fastener continuously from the drum surface. Each of the mold discs has in one side surface a multiplicity of hook-forming cavities spaced circumferentially at regular distances and extending from the circumferential surface toward the center. Opposite side surfaces of each of the spacer plates are smooth and flat.
Regarding the molding technologies disclosed in the U.S. Pat. No. 3,312,583 and International Patent Japanese Publication No. HEI 1-501775, though the two technologies are different from each other in means for forcing the resin into the hook-forming cavities, hooks, which have been molded in the cavities integrally with the substrate, are removed from the drum surface together with the substrate in timed relation with rotation of the drum while the spacer plates are fixed. These molding technologies are simple in structure and process, as compared to the molding technology of Japanese Patent Publication No. SHO 48-22768.
The reason why the spacer plates must be used in the prior art is that the whole contour of the individual hooks could not have been made in a single mold. Further, partly since the cavities to be provided in the mold disc must be such that distal ends of hooks must be oriented in the circumferential direction of the disc, and partly since the shape of side surface of the rising portion must be simple, the individual hooks tend to fall flat from their base as they are slender. Because of such restriction of structure, the books must be oriented in a common circumferential direction so that it would be difficult to secure necessary engaging forces in all directions. Further, since after repeated use the hooks would fall flat and would not restore their original standing posture, their engaging rate with loops of the companion fastener memeber would lower so that the continuation of the engaging forces could not be expected. Under these circumstances, the present inventors proposed to form a reinforcing rib on at least one side surface of the rising portion of the individual hook in order to prevent the hook from falling flat and also to make the orientation of the hooks opposite between adjacent rows in order to distribute plural engagement directions of the hooks (Japanese Utility Model Laid-Open Publication No. HEI 4-31512).
However, with the engaging members of the prior art surface fastener in which the substrate and the hooks are molded integrally, because of technological difficulty in making a mold, it is impossible to obtain a delicate shape as the woven cloth type, and the degree of toughness of the hooks and the substrate are too low if the hooks have the same size as that of monofilaments, thus making the surface fastener not suitable for practical use. Consequently, the size of the individual hooks must be increased in order to secure a desired degree of toughness. Further, the reinforcing ribs are provided so that the surface fastener becomes rigid.
If the hooks are rigid, the substrate for pulling off the hooks from the cavities after molding must have the same toughness. Therefore, it is inevitable to increase the thickness of the substrate to match with the toughness of the hooks during molding so that the whole surface fastener would become much more rigid, thus making the surface fastener difficult to be used with an article that needs adequate flexibility.